Line large baking sheet with waxed paper. Using wet hands, shape 1/4 cup fish mixture for each dumpling into egg-shaped oval. Place on prepared sheet. Cover with waxed paper and chill while preparing cabbage and steamer.

Set vegetable steamer rack in large pot. Fill pot with enough water to meet, but not cover, bottom of rack. Line rack with cabbage leaves. Arrange 8 fish dumplings on leaves; cover with additional cabbage. Bring water to boil. Cover pot and steam dumplings until cooked through and firm to touch, about 25 minutes. Transfer upper cabbage leaves to platter. Top with cooked dumplings. Cover with bottom cabbage leaves. Steam remaining dumplings in additional cabbage leaves in 2 more batches. Cover and refrigerate gefilte fish until cold, at least 6 hours. DO AHEAD Can be prepared 2 days ahead. Keep refrigerated.

sauce

Push garlic through garlic press into small bowl or mince garlic and place in small bowl. Mix in horseradish and lemon juice. Gradually whisk in mayonnaise. Season sauce with salt and pepper. Cover; chill up to 1 day.